Height adjustment mechanisms for the backrest of work chairs are known in many forms. It follows that the parts on which the spinal column of the user is supported in sitting and/or leaning back can easily be adjusted to a height position relative to the seat satisfying the requirement of the individual user. Such support and adjusting mechanisms must naturally support and transmit the forces which are to be transmitted from the backrest to the support arm and thereby to the chair frame and indeed also when the backrest is obliquely loaded. On account of this height adjustment, a single support arm is as a rule selected. Thereby torsional forces must also be supported. It is important that such height adjustment is easy to actuate and does not become jammed even after a long time so that one can, if possible, free the arresting mechanism and with the same hand effect the height adjustment. With most known mechanisms that is not possible. Height adjustment mechanisms provided with a screw have the advantage that step-less adjustment can be made; however, they require actuation with both hands. Adequate fine-step height adjustment can also be created which serves the practical need. In order suitably to support the forces, relatively long guides are necessary. These are inclined to stick.